Playing with Digital Elevation Models

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retired jerry
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Re: Playing with Digital Elevation Models

Post by retired jerry » March 18th, 2018, 11:23 am

once you set it up, can you incorporate updates easily? or do you have to do the entire process?

updating because you can is good :)

if there was a landslide or volcanic eruption you'd want to do an update :twisted:

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aiwetir
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Re: Playing with Digital Elevation Models

Post by aiwetir » March 18th, 2018, 2:14 pm

Since I think it would all be done on a web server, it could all be automated. It just depends on how much money you have and how often you want to update it
- Michael

RobFromRedland
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Re: Playing with Digital Elevation Models

Post by RobFromRedland » March 25th, 2018, 11:53 am

aiwetir wrote:
RobFromRedland wrote:I'm curious - where can you find public LIDAR data?
https://gis.dogami.oregon.gov/lidarviewer/

It's a little unintuitive, but you have to have the Downloadable Lidar Data layer on, click on a layer and then dig into the menu sometimes.
I'm quite a newbie with QGIS - I took an intro course on Coursera a few months ago using ArcGIS, but my license expired, so I'm using QGIS now.

I downloaded a couple of the lidar datasets from the above website, however I'm having a hard time importing it into QGIS. Can you point me in the right direction? I unzipped them into their own folders and attempted several different ways, and all get an "invalid datasource" error.

I know this isn't a technical forum, but others might find it interesting to know how to import the lidar data to do some experimenting. The data for areas I'm looking is limited, but there are a few places I could do some looking - I'm just trying to see if old trail bench shows up on these lidar scans.

Thanks
Life is not a journey to the grave with the intention to arrive safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming: WOW-What a ride!

Titogoeshiking3
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Re: Playing with Digital Elevation Models

Post by Titogoeshiking3 » March 27th, 2018, 1:56 pm

aiwetir wrote:Started messing with Lidar DEMs in school last week and it occurred to me that there are a lot of popular places with crappy topo maps out there. Saddle Mountain, Multnomah Falls and the one to the west of that. So I threw this together tonight. I even traced out the trail because you can clearly see it in the bare earth hill shading. Lidar is a magic tree eraser. Anyway here's one map, I'll throw some together next week after finals. Will take requests, may compile them onto one site if people show any interest.

Oh, hey, regular disclaimer, don't go walking off a cliff because you are using my map.

Image

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aiwetir
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Re: Playing with Digital Elevation Models

Post by aiwetir » March 30th, 2018, 9:51 pm

RobFromRedland wrote:
I'm quite a newbie with QGIS - I took an intro course on Coursera a few months ago using ArcGIS, but my license expired, so I'm using QGIS now.

I downloaded a couple of the lidar datasets from the above website, however I'm having a hard time importing it into QGIS. Can you point me in the right direction? I unzipped them into their own folders and attempted several different ways, and all get an "invalid datasource" error.

I know this isn't a technical forum, but others might find it interesting to know how to import the lidar data to do some experimenting. The data for areas I'm looking is limited, but there are a few places I could do some looking - I'm just trying to see if old trail bench shows up on these lidar scans.

Thanks
Dig into the bare_earth folder, all the way down in the folders and look for the hdr.adf file. Drag that into QGIS and you should be good. I'm learning ArcMap but I'm self-learning QGIS, basically redoing all my class work in QGIS when I can. There are still things that I'm struggling with with the coordinate reference system with QGIS that don't work at all like ArcMap.

Once you have that in, Raster > Analysis > Hilllshade (315° / 45°) will give you something you can look at. You can just change the hdr.adf layer style to "Singleband pseudocolor" (BrBG then invert it) to give it some height color too. If that's too vague, just ask again and I'll walk you through it.
- Michael

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aiwetir
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Re: Playing with Digital Elevation Models

Post by aiwetir » March 30th, 2018, 10:01 pm

I'm still working on this one too, just very slowly. Mostly trying to make it into something someone could print or use in real life

Image
- Michael

RobFromRedland
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Re: Playing with Digital Elevation Models

Post by RobFromRedland » March 31st, 2018, 10:11 am

aiwetir wrote: Dig into the bare_earth folder, all the way down in the folders and look for the hdr.adf file. Drag that into QGIS and you should be good. I'm learning ArcMap but I'm self-learning QGIS, basically redoing all my class work in QGIS when I can. There are still things that I'm struggling with with the coordinate reference system with QGIS that don't work at all like ArcMap.

Once you have that in, Raster > Analysis > Hilllshade (315° / 45°) will give you something you can look at. You can just change the hdr.adf layer style to "Singleband pseudocolor" (BrBG then invert it) to give it some height color too. If that's too vague, just ask again and I'll walk you through it.
The Hillshade option in the Style/Render Type got me so I could see the data. I'm still kind of in the dark as to what all those options mean, but at least I've made some progress. Unfortunately, there isnn't much data for most of the areas I'm really interested in.

If you have any good QGIS tutorials or anything you've found I'd love to know. I've not found a good one yet - especially for someone new to GIS concepts in general.

Thank you!
Life is not a journey to the grave with the intention to arrive safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming: WOW-What a ride!

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aiwetir
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Re: Playing with Digital Elevation Models

Post by aiwetir » March 31st, 2018, 4:39 pm

What version are you using?
- Michael

RobFromRedland
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Re: Playing with Digital Elevation Models

Post by RobFromRedland » March 31st, 2018, 5:06 pm

aiwetir wrote:What version are you using?
I've been using both 2.14.16 and the new 3.0 version.
Life is not a journey to the grave with the intention to arrive safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming: WOW-What a ride!

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aiwetir
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Re: Playing with Digital Elevation Models

Post by aiwetir » March 31st, 2018, 8:45 pm

In QGIS 3, you have your DEM in there as be_XXXXXX
Raster > Analysis
The band number only applies if you have a color image or greater (going into IR or radio spectrums)
The Z factor is vertical exaggeration
Scale is ratio of vertical units to horizontal, kinda like Z factor but the other way.
Azimuth of the light is just where the sun is shining from
Altitude is the same. 315/45 is almost always a nice rendering if you move it around too much people see things backwards, like looking into the back of a face mold.
The rest you can ignore.


Other things you can do with DEM Rasters I didn't proof this very well

Elevation Tint

• Add the “Layer Styling” panel to your display by going to the Main Menu bar and selecting and turning on View > Panels > Layer Styling. The Layer Styling panel should open on the right of your QGIS window.
• The DEM came in with a “Singleband grey” appearance, meaning that the elevation values are rendered using only shades of grey.

We want to apply a “color ramp” that uses diverging colors to represent elevation values. This is also sometime referred to as an “elevation tint”.

• In the “Layer Styling” panel, change the “Symbology” dropdown from “Singleband grey” to “Singleband Pseudocolor”.
• Next, change the “Color” dropdown to “BrBG” and check the “Invert” check box on.
(Note: if your DEM is not displaying after performing this step, you may need to click the
“Classify” button)
• Check your results with the below example. Now the DEM is using a more natural color gradient that “looks” like terrain. This is simply for visualization purposes.


================================

Slope Analysys

• From the Menu Bar in QGIS, select Raster >Terrain Analysis > Slope. This will bring up the
“Slope” geoprocessing tool dialog window. From here, set the following fields:

7
Elevation layer: Your DEM
Output layer: Save to your working directory on your
computer as “whatever you want.tif”
Output format: GeoTIFF
Z factor: 1
Add result to project: Checked

Select “OK” when ready. The slope process will execute. You will not
receive any indication it is complete but it is successful when the tool dialog window
closes and the slope loads into the map canvas.


===========================================

Contour Lines

From the Menu Bar in QGIS, select Raster > Extraction > Contour. This will bring up the
“Contour” geoprocessing tool dialog window. From here, set the following fields:

Input file (raster): Your DEM
Output file for contour lines (vector): Save to your working directory on
your computer as “whatever.shp”
Interval between contours: many maps use 40', make sure your CRS is in feet if you want feet
Attribute name: Checked (adds elevation to attribute table)
Load into canvas when finished: Checked

The contour process will execute. You will not receive any indication it is complete but it is successful when the tool dialog window closes (close it if it doesn’t on its own!) and the contour loads into the map canvas.
- Michael

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